by Paul Doherty and Linda Shore
Ever notice that when it’s humid
you have a Bad Hair Day? Hair
increases its length when humidity
increases. So curly hair frizzes and
straight hair goes limp. From dry to
humid, hair length can change by 3
percent.
In this activity, we’ll hook up a hair
to a lever system and create a hair
hygrometer to measure changes in
humidity. Invented in 1783, the hair
hygrometer was so reliable that it
was not replaced by an electrical
instrument until the 1960s.
Materials
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Corrugated cardboard about
a foot long and 9 inches
wide
A piece of thin cardboard
A pair of scissors
A knife
A pushpin or other largediameter pin
A straight or common pin
A strand of hair that's one
foot long
A hot glue gun
A dime
To Do and Notice
1. On the top edge of the
corrugated cardboard, cut two slits
about 1/4 inch apart and 1 inch
from the left side.
2. From a piece of thin cardboard,
cut out a triangular pointer about 6
inches long.
3. Cut two slits along the bottom of
the pointer about 1 inch from the
left edge.
4. Attach the pointer to the base
with a pushpin 1/2 inch from the
left edge in the center. Then take
the hair strand and slide it through
the top two and bottom two slits.
5. Hot glue the hair in place in both
sets of slits, then hot glue a dime
11/2 inches from the left edge of
the pointer base.
6. Push a pin through the hole in
the pointer so that the hair is
slightly stretched when the pointer
is horizontal.
7. Calibrate your hair hygrometer
at 100 percent humidity by bringing
it into the bathroom when you
shower. Make a mark when the
pointer stops. Then use a hair
dryer to dry the hair and make a
mark for 0 percent humidity.
8. Monitor the changing position of
your pointer as the humidity
changes.
What's Going On?
Hair is made from keratin, a protein that is wound into a coil. The turns of
the coil are held together by a type of chemical bond called a hydrogen
bond. Hydrogen bonds break in the presence of water, allowing the coil
to stretch and the hair to lengthen. The bonds re-form when the hair
dries, which allows people to style their hair simply by wetting it, shaping
it, then drying it.
Material
A strand of hair
To Do and Notice
1. Wrap the strand of hair
around your forefingers so
that your fingers are about
an inch apart.
2. Pull the hair slowly, but
firmly.
3. Can you feel the hair
stretching out?
Material
A strand of straight hair
To Do and Notice
1. Hold up a single strand of hair. Pinch
the hair between the fingernail of your
thumb and the fleshy part of your
forefinger.
2. Run your fingernail across the hair
strand. Try not to break the hair in two. If
you do, get another hair and try again.
Did your hair curl? If not, try again. Be
patient; you'll get the hang of it.
What's Going On?
Healthy head hair will
stretch when you pull on it.
That's because the cortex of
healthy hair is strong and
elastic. The cortex inside of
a damaged hair strand is
weak and brittle. It will break
more easily when you pull
on it.
What's Going On?
Okay, so this might not be the most
efficient way to style your hair. But this
activity wouldn't be possible if it weren't
for the way the cuticle of a hair is
structured. Your cuticle is made of
overlapping cells arranged like the tiles on
a terra-cotta roof. When you run your
fingernails across a hair strand, you pull
apart the overlapping cells on one side of
the cuticle more than you pull apart the
cells on the opposite side. The result: a
tiny hair that kinks, twists, and curls.
© Exploratorium